Batboard

Hi all,

Sorry for the basic questions, but what does this option enable my radio to do apart from the obviuos encryption function, what does OFB mean? it also has W969AF Multikey. This is an IMBE Radio.

Many Thanks Steve

OFB and CFB are two different types of DES encryption.

When you do DES, you take a 56 bit number, mash on it several times, and get another 56 bit number. You then use that 56 bit number to encode your digitally represented voice.

Now, you obviously want to keep changing that number, or you aren't very secure. So you keep re-running the DES algorithm, feeding "something new" in each time.

So, what do you feed in? You have 2 real choices:

You can take the 56 bit number you got out of the DES encoder last time and feed it back in - this is called Cypher FeedBack or CFB. The advantage to this approach is that if you receive a corrupted bit over the air it will only screw up the current block of data. The disadvantage is that it is eaiser to crack.

Or you can take the encrypted voice data and feed that back into the next round of DES - this is Output FeedBack or OFB. The advantage is this is harder to crack, but the disadvantage is that if you get a corrupted bit in the data stream, that corruption will continue to affect you until you somehow resync. That is why things like APCO-25 transmit a low-speed data stream that every once in a while is used instead of the voice stream (this is called the MI record).

Are you sure that you don't have CFB and OFB confused with each other?

You are correct (Serves me right for trying to multitask too much).

OFB wraps the output of the DES routine back on itself, CFB wraps the encyphered text back on itself.

Brain fart - my bad.

Take what I said and reverse it.

(Should have looked at the source code rather than trusting my memory...)

Here is a great PDF document which describes the 5? different modes of DES and how each of them work (into details that scare me):

isl.cudenver.edu/lkurgan/CSC4811/s03/class9.pdf

Lindsay

Cheers for the valued info.

Regards Steve